Experience sharing of cold forging forming process

1. The concept of cold forging

The higher the temperature, the softer the metal material becomes, the easier it is to warm forging. Since the oxidation of the steel surface is rapidly accelerated at 900°C or more, the accuracy of the fastener at high temperature is poor. In addition, cold forging at low temperature, forging precision is good, strength is also high, cold heading forging forming pressure is high, whether it is a bolt or nut, in the cold heading forging are easy to cause cracking.

In metalology, the forging of steel heated to above the recrystallization temperature (about 700°C) is called warm forging, and the recrystallization temperature is hereinafter called cold heading or cold forging. In fact, generally according to the operating conditions, according to the forging temperature and the usual name, the forging name of the subdivision. Traditionally, it is difficult to imagine that the heating forging below recrystallization belongs to cold forging. When the material is forged at room temperature, it is called (narrow) cold forging. When the material is heated and forged, it is called warm forging. In addition, the forging with little surface oxidation at about 850 ℃ is also called warm forging in practice. Only when the forging is heated and formed above 950 ℃ is broadly called forging (red dozen).

2, cold heading or cold forging method and process

End face correction and upsetting are simple compression deformation, which are used to correct the cut surface and adjust the shape of the material. Semi-hermetic forging is used for high precision forging. The main process of cold forging is the extrusion of the shaft and the cup, and the extrusion process can obtain high precision in the radial direction. The punching process is used to cut off unwanted parts of the skin or flash.

It can be said that cold forging is a combination of various processes to obtain the final shape of the fastener. For example, for an axle nut of cold forging, the process is: blank cutting → shaft rod positive extrusion → cup cylinder reverse extrusion → cup cylinder positive extrusion → upsetting → punching → positive extrusion, etc. It is divided into multiple processes to avoid excessive pressure during one forming. However, the less the process, the lower the cost. Reducing the forming pressure and reducing the number of processes is the key to the process design, and it is also inevitable to reduce the cost.

3. Basic technology of cold heading or cold forging

① Deformation resistance of materials

The strength of the material during the cold forging process is expressed as deformation resistance. The deformation resistance increases as the degree of processing (strain) increases, and this phenomenon is called work hardening. When the material is compressed to the original height of 1/3 when the deformation, the strain rate is 1.0, the processing pressure, forming tonnage and deformation resistance is proportional.
In carbon steel, the deformation resistance of 45# steel is about 1.5 times that of 10# steel, so the forging of low carbon steel is easier. ML35CrMo, ML40Cr low alloy steel and 45# carbon steel deformation resistance close; and 06 Cr19Ni10(0 Cr18Ni9, SUS304 stainless steel) deformation resistance is higher.

② Die life

When the pressure of the mold is too large, it will lead to cracking, even if the force can not crack once, repeated loading will lead to fatigue damage. Forging forming force will affect the number of fasteners (fatigue life) that a mold can process. From the relationship between the pressure and life of LD steel hexagon socket head punch, it can be seen that the compressive failure strength of the punch is above 2500MPa, but the die life is only a few thousand pieces under 2500MPa pressure, 30000 pieces under 2000MPa pressure and more than 50000 pieces under 1500MPa pressure. The pressure reduction of the mold can greatly improve the life of the mold.

③ Die surface pressing and forging tonnage

The forging pressure Pm is proportional to the deformation resistance Y and can be expressed by the following formula: Pm = CY
C is the constraint coefficient, which represents the degree of deformation of the constrained material. When the constraint is small, the C value is 1~2, and when the constraint is large, the C value will exceed 4.
If the projected area of the contact part between the material and the pressurizing die is set as a, the forging tonnage p is the product of Pm and a, as shown in the following formula: P = AφPm = CAY
The deformation resistance changes with the strain, and the change decreases after the strain is greater than the 0.5. In the approximate calculation, the deformation resistance value at the time of strain 1.0 is substituted as the Y value for calculation. For example, the deformation resistance of 45# steel is approximately Y = 720PMa, and 9.8PMa = 1Kgf/mm is set here.².

④ Processing heat

The temperature of the fastener after cold heading or cold forging is generally increased by 100~200 ℃. This is because most of the forging energy is converted into heat. During continuous cold heading or forging, the increase in the temperature of the workpiece will cause the increase in the temperature of the die, which will affect the accuracy of the fastener product and should be noticed. Such as diameter M20 bolts, the relationship between the displacement of the mold and the temperature change of the material surface can be seen, the processing time is about 0.15S, and the temperature rises about 150 ℃.

⑤ Steel for cold forging

Cold heading or cold forging processing will get different shapes according to different processing procedures: ① wire rod → annealing → lubrication → cold forging forming → ② bar stock → annealing → drawing → lubrication → cold forging forming → recrystallization annealing.

Cold forging is generally carried out on horizontal cold heading or forging machines such as cold heading machines and high-speed cold forging machines.

Usually used in cold heading or cold forging steel, commonly used low and medium carbon structural steel, low alloy structural steel, in GB/T699-1999 "high quality carbon structural steel" and GB/T3077-1999 "alloy structural steel. These materials are not special steel for cold forging, but try to use the grades in GB/T6478-2001 "Cold Heading and Cold Extrusion Steel" standard. When used for cold heading or cold forging, there are no surface defects, low deformation resistance, and the number and shape of non-metallic inclusions are very small, which can inhibit cracking.